Doesn't sound like she's quite ready to settle down yet, She's right, there will come a time when you both will feel attracted to other people that's normal, just because you've done your shopping you don't stop looking in shop windows, that is a realistic view however the thing you are rightly concerned about is she has basically said what she will do if the relationship runs into difficulties, I wonder if there is already someone at work she is attracted to and she is therefore especially aware of the risk, I wonder if relationship counselling is worth trying sooner rather than later in your case.
She may also be getting confused feelings about the whole moving in and settling down thing, personally I would hold off for a while until you have worked this issue through. I feel if she does stray later on; and as she seems to take it for granted this will happen it is possible, her defence is already in place in that she warned you in advance and you still wanted to get married or whatever.
Good luck hope it works out for you